Asynchronus communication system for remote monitoring of objects or an environment

ABSTRACT

A system provides for remote monitoring using asynchronous code division multiple access (CDMA) communication techniques between a base station and one or more transponders. Each transponder is attached or otherwise associated with an object or an environment to be monitored. Upon receipt of an interrogation signal, a transponder generates and transmits a coded transponder signal containing the monitored data. The coded transponder signal is generated using a spreading code. Each transponder is associated with a unique, mutually exclusive set of spreading codes. Each spreading code is based on a unique transponder address and the monitored data to be sent. A detector asynchronously monitors received signals for any of the available spreading codes. Once the detector detects a particular spreading code, the base station can identify the source transponder and extract the monitored data.

TECHNICAL FIELD

The invention relates to communication systems and, more particularly,to remote monitoring systems.

BACKGROUND

There are many applications for monitoring objects or environments fromremote locations. Such applications include identification andmonitoring of objects such as files, records, books, equipment or otherarticles; identification and monitoring of biological tissue; oridentification and monitoring of animals, insects or people. Otherapplications include remote monitoring of status information or sensorreadings associated with each object to be monitored, such as biomedicalinformation, information concerning whether an object has been moved orotherwise tampered with, or other status information concerning anobject. Still other applications include remote monitoring of anenvironment for parameters such as movement, heat, light, sound, weatherrelated parameters, or presence of airborne particles or chemicals.

One type of remote monitoring system includes a set of transponders thatare attached or otherwise associated with the target objects orenvironments to be monitored. Each transponder carries information aboutthe object or the environment with which it is associated. Upon receiptof an interrogation signal, the transponder generates and transmits aresponse. A base station interrogates the transponders, the transpondersgenerate and transmit their responses, and the base station receives andinterprets their response.

In some applications, objects are to be monitored in environments wherea line-of-sight between the transponder and the base station may notalways be available. Such environments may include open-field, forested,mountainous or aquatic outdoor environments in which obstructions suchas trees, hills, waves or other variations may potentially interferewith communication. In urban and indoor environments, the obstructionsmay be buildings, walls, furniture or vehicles.

SUMMARY

In general, the invention provides a system for remote monitoring ofobjects or an environment. In operation, an interrogator sends out aninterrogation signal at a given frequency. The interrogation signalactivates each of a set of transponders within range of theinterrogation signal. In response to the interrogation signal, eachtransponder generates and transmits a coded transponder signal. In oneembodiment, each transponder is associated with a unique, mutuallyexclusive set of spreading codes. Each spreading code is based on aunique transponder address and the data to be sent.

After interrogation, a detector asynchronously examines incoming signalsfor presence of any coded transponder signals. Once presence of aparticular spreading code determined, the detector may use the knownallocation of spreading codes to identify the source transponder and toidentify the data conveyed. The detector identifies, from the knownallocation of spreading codes, which transponder sent each detectedcoded transponder signal as well as the data conveyed by each detectedcoded transponder signal.

The invention may provide one or more advantages. For example, thesystem allows for asynchronous communication between a detector and thetransponders. Spread spectrum communication permits multipletransponders to transmit simultaneously using the same frequency with areduced likelihood of interference. The absence of synchronicityconstraints permits the transponders to respond asynchronously andintermittently or in a pulsed manner, and reduces or eliminates the needfor handshaking or other types of timing techniques that may otherwisebe required for synchronous communication. The asynchronouscommunication techniques described herein may therefore reduce the size,complexity, and power requirements of the transponder hardware.

In one embodiment, the invention is directed to a transponder comprisinga clock generator that generates a transmit clock based on aninterrogation signal, a code generator that generates a spreading codefrom a set of spreading codes allocated to the transponder using thetransmit clock, and a signal generator that transmits a transpondersignal that includes the spreading code.

In another embodiment, the invention is directed to a system comprisinga set of one or more transponders, wherein each transponder comprises aclock generator that generates a transmit clock based on aninterrogation signal, a code generator that generates a spreading codefrom a set of spreading codes allocated to the transponder using thetransmit clock, and a signal generator that transmits a transpondersignal that includes the spreading code, and a detector that acquiresreceived signals and examines the received signals to detect presence ofone or more spreading codes.

In another embodiment, the invention is directed to a method comprisingsending an interrogation signal sufficient to enable communication withone or more transponders, and asynchronously examining received signalsamples to detect presence of one or more of a plurality of spreadingcodes sent by the transponders.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of the system for remote tracking ormonitoring in accordance with an embodiment of the invention.

FIG. 2A is a block diagram of a transponder of the system of FIG. 1 forremote tracking or monitoring, and FIG. 2B is a block diagramillustrating one embodiment of a clock generator.

FIG. 3 shows a block diagram illustrating another embodiment of atransponder that includes multiple code generators.

FIG. 4 is a block diagram illustrating another embodiment of atransponder.

FIG. 5 shows a digital logic implementation diagram of a code generator.

FIG. 6 is a flow diagram illustrating exemplary operation of atransponder generating and sending a coded transponder signal to basestation.

FIG. 7 is a block diagram of an interrogator of the system of FIG. 1 forremote tracking or monitoring.

FIG. 8 is a block diagram showing one embodiment of a detector of thesystem of FIG. 1 for remote tracking or monitoring.

FIG. 9 is a block diagram illustrating an exemplary code allocationlookup table maintained by a base station.

FIG. 10 is a flow diagram illustrating exemplary operation of a detectordecoding a coded transponder signal.

FIG. 11A illustrates a graph of a received signal sample and FIG. 11Billustrates a graph of a normalized correlation of the received signalshown in FIG. 11A.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a system 10 for remote monitoring.System 10 includes a base station 14 and a set of transponders 12A-12N(“transponders 12”), each attached or otherwise associated with antarget object to be monitored, or placed in a target environment to bemonitored. System 10 may, for example, be used to monitor statusinformation associated with animals, insects or people. System 10 mayalso be used to monitor status information of objects such as files,records, books, equipment, biomedical materials, or articles in awarehouse, retail store or other environment. In addition, system 10 maybe used to monitor status information of a target environment. It shallbe understood, therefore, that the invention is not limited with respectto the target object, target environment, or specific parameters to bemonitored, and that the invention can be used for virtually anyapplication in which remote monitoring is desired.

In the embodiment shown in FIG. 1, base station 14 includes aninterrogator 16 and a detector 18. In another embodiment, interrogator16 and detector 18 are not co-located in base station 14, but may belocated separately. In another embodiment, more than one interrogator 16and/or more than one detector 18 may be deployed. Use of multipledetectors 18 may increase the range of system 10. It shall beunderstood, therefore, that alternative embodiments other than thespecific embodiment of system 10 shown in FIG. 1 may be used withoutdeparting from the scope of the present invention.

Interrogator 16, detector 18 and the transponders 12 communicate viawireless signals indicated generally in FIG. 1 as signals 15A-15N. Inone embodiment, the signals 15A-15N are radio frequency (RF) signals. Itshall be understood, however, that other frequencies could also be usedwithout departing from the scope of the present invention.

In operation, interrogator 16 sends out a generic interrogation signalat a given frequency. In response to the interrogation signal,transponders 12 generate and transmit a coded transponder signal. In oneembodiment, transponders 12 generate the coded transponder signal usingspread spectrum code division multiple access (CDMA) techniques. Thecoded transponder signal is generated using a spreading code andincludes transponder identification information and monitored datainformation. Each transponder may be associated with a unique, mutuallyexclusive set of spreading codes. Each spreading code is based on aunique transponder address and the monitored data to be sent.

Interrogator 16, transponders 12 and detector 18 communicateasynchronously. Thus, interrogator 16, transponders 12 and detector 18have no information from each concerning the time-of-arrival nor thephase of either the interrogation signal or the coded transpondersignals. Variations in the time-of-arrival of a coded transponder signaldue to disparate distances between the base station and individualtransponders or due to variable transponder activation latency are alsonot controlled or known. Detector 18 asynchronously examines receivedsignal samples to detect presence of any of the available spreadingcodes. Detector 18 samples a received signal and performs a slidingcorrelation of the received signal with a series of ideal correlatorsplaced at every possible phase shift along the time axis. Once thedetector detects a particular spreading code, it may identify the sourcetransponder and extract the monitored data using the known allocation ofspreading codes.

The use of spread spectrum communication between interrogator 16,transponders 12, and detector 18 allows multiple transponders 12 totransmit simultaneously using the same transmit frequency with reducedinterference. Also, transponders 12 may send data to base station 14intermittently or in a pulsed manner. Furthermore, use of theasynchronous CDMA transmission techniques described herein may reducethe size, complexity, and power requirements of transponders 12.

FIG. 2A shows a block diagram of one embodiment of a transponder 12.Operation of a single transponder 12 will be described for exemplarypurposes. It should be noted, however, that each of transponders 12 insystem 10 operates in a similar fashion. Transponder 12 includestransponder receive antenna 20, threshold/trigger circuit 22, clockgenerator 23, code generator 28, ID/data storage 30, transponder signalgenerator 24, and transponder transmit antenna 34. In one embodiment,transponder 12 is a passive transponder, drawing a small amount of poweruntil it is activated by an interrogation signal from interrogator 16.Threshold/trigger circuit 22 detects receipt of an interrogation signalvia transponder receive antenna 20 and “wakes up” the rest of thetransponder circuitry.

Clock generator 23 generates a transmit clock based on the interrogationsignal received from base station 14. Clock generator 23 may, forexample, comprise a clock divider that generates a transmit clock havinga frequency that is a defined fraction of the frequency of theinterrogation signal. In one embodiment, clock generator 23 may be adivide-by-n circuit, where n is any integer, that generates a clock thatis 1/n the frequency of the interrogation signal. For example, clockgenerator 23 may be a divide-by-two circuit that generates a transmitclock having a frequency of one half the frequency of the interrogationsignal. In this example, an interrogation signal of 10 GHz would resultin a 5 GHz transmit clock. In other embodiments, the clock generator 23may be implemented as a clock multiplier rather than a clock divider,such as a multiply-by-n circuit. In another embodiment, clock generator23 may generate a transmit clock having the same frequency as theinterrogation signal. For example, the system 10 could be designed as asimultaneous transmit/receive system, i.e., a system in which theinterrogating and transponder signals are transmitted at the samefrequency. In that embodiment, transponder 12 and detector 18 mayinclude discrimination circuitry to differentiate the interrogating andtransponder signals in a manner well known in the art. It shall beunderstood, therefore, that the actual frequency generated by clockgenerator 23 may vary depending upon the particular implementation orapplication, and that the invention is not limited in this respect.

In one embodiment, clock generator 23 may be implemented using aprogrammable counter using the interrogation signal as its input. Oneexample of such an embodiment is shown in FIG. 2B. In FIG. 2B, aprogrammable counter 29 is connected to receive the interrogation signal(INT) at its input. In the example of FIG. 2B, programmable counter 29is a modulo-16 counter having outputs z₀-z₃ indicated generally byreference numeral 25. Each pulse of the interrogation signal causes theoutput of programmable counter 29 to increase by 1. In this way,programmable counter 29 “counts” pulses of the interrogation signal. Theoutput pulses along each output z₀-z₃ of programmable counter 29 arethus directly related to the frequency of the interrogation signal. Forexample, for a modulo-16 counter, pulses appearing at output z₀ havehalf the frequency of the interrogation signal; pulses appearing atoutput z₁ have one-quarter the frequency of the interrogation signal,etc. Programmable counter 29 thus acts as a frequency divider whenimplemented as clock generator 23. Clock generator 23 may also beimplemented using any divide-by-n frequency divider, any multiply-by-nfrequency multiplier, or may generate a clock having the same frequencyof the interrogation signal.

By generating the transponder transmit clock based on the interrogationsignal, several advantages may be obtained. For example, transponder 12is not required to generate its own clock, which may reduce transponder12 overall size and power requirements. In addition, transponder 12could be used over a frequency range determined by the filter 62 (seeFIG. 4). This allows flexibility on the frequency transmitted byinterrogator 16.

Referring again to FIG. 2A, code generator 28 generates one of thespreading codes from the set of spreading codes allocated to transponder12. Code generator 28 generates the spreading code based on transponderidentification information and on the monitored data information. Inparticular, code generator 28 inputs a set of address bits and a set ofdata bits from ID/data storage 30 and outputs an n-bit spreading codebased on the address and data bits. In one embodiment, each transponder12 is assigned a unique address. This unique address forms part of then-bit seed for the generation of the spreading code and serves to definea unique set of spreading codes allocated to each particulartransponder. For example, if transponder 12 is assigned k address bits,which remain fixed, then the set of spreading codes assigned totransponder 12 contains all possible spreading code combinations with kaddress bits set with the unique transponder address and (n-k) bitsavailable as data bits. Thus, for a 9-bit spreading code in which threeof the bits are address bits, each transponder 12 would be allocated aset of 64 (2^(n-k)=2⁹⁻³=2⁶) mutually exclusive spreading codes.

The number of spreading codes allocated to each transponder 12 maydepend on the number of transponders deployed in the system. Forexample, for a fixed n-bit code generator seed, the number of bits kassigned to the unique transponder address may vary depending upon thenumber of transponders deployed. If fewer transponders are required,fewer bits may be assigned as address bits, more bits may be availablefor data transmission, and each transponder may be allocated acorrespondingly larger set of spreading codes. Similarly, the larger thenumber of transponders 12 in the system the smaller the set of spreadingcodes allocated to each transponder 12. It shall be understood,therefore, that the precise number of bits allocated for address and fordata may be varied depending upon the number of transponders in thesystem, as well as upon other factors, and that the invention is notlimited in either the number of transponders deployed or in theparticular allocation of address and/or data bits.

Although the address bits associated with each transponder 12 and inputinto code generator 28 are fixed for each transponder, the data bits mayvary depending upon the data to be sent. The particular spreading codegenerated by code generator 28 from the set allocated to transponder 12is therefore dependent upon the data bits. In other words, for the 9-bitspreading code discussed above in which three of the bits are addressbits and the six remaining bits are data bits, the allocated set of 64mutually exclusive spreading codes corresponds to 64 different datawords available to each transponder. Generation and detection of thecoded transponder signals is described in further detail below.

ID/data storage 30 may be implemented using any number of storagedevices known in the art, such as electrically erasable programmableread only memory (EEPROM), random access memory (RAM), read-only memory(ROM), non-volatile random access memory (NVRAM), flash memory, magneticor optical media, or the like.

Transponder signal generator 24 modulates the carrier frequency of thecoded transponder signal output by code generator 28, provides filteringand amplification of the coded transponder signal, and transmits thecoded transponder signal for receipt by the base station 14 viatransponder transmit antenna 34. In some embodiments, receive antenna 20and transmit antenna 34 may be integrated as a single antenna.

The frequency or set of frequencies with which the transponders 12,interrogator 16 and detector 18 communicate may vary depending upon theapplication for the system and the associated environment, desiredtransponder size and power requirements. In one embodiment, theinterrogation signal is approximately 10 GHz, with a corresponding 5 GHzcoded transponder signal. This embodiment enables use of very smalltransponder receive and transmit antennas 20 and 34, respectively,appropriate for those applications for a transponder 12 having anextremely small form factor. The 10 GHz/5 GHz frequency set allowsremote tracking and/or monitoring of objects over a range ofapproximately 2 km.

In another embodiment, the interrogation signal is tuned to 450 MHz witha corresponding 225 MHz transponder signal. This embodiment, whilerequiring a slightly larger transponder receive and transmit antennas,20 and 34, respectively, achieves a greater range over which thetransponders 12 can communicate with interrogator 16 and detector 18 andcan also enable improved foliage, ground, aquatic and buildingpenetration for applications where a line-of-sight is not alwaysavailable. The 450 MHz/225 MHz set of frequencies allows remote trackingand/or monitoring of objects over a range of at least 25 kilometers.

The design of the transponder 12 may allow for a reduction in physicalsize, complexity and power requirements. In one embodiment, for example,the transponder 12 has a size of less than 5 mm in its longestdimension. In another embodiment, the transponder 12 has a size of lessthan 1 mm in its longest dimension. In another embodiment, thetransponder 12 has a size of less than 0.75 mm in its longest dimension.In addition, in one embodiment, the transponder 12 has a weight of lessthan 20 milligrams. In another embodiment, the transponder 12 has aweight of less than 10 milligrams.

The data transmitted by transponder 12 can include status information orother data concerning the transponder 12 itself, an object with whichthe transponder is associated, or an environment in which thetransponder is located. This status information may be obtained fromsensor readings received from sensor 32. In the embodiment illustratedin FIG. 2A, code generator 28 is connected to receive status informationfrom a sensor 32. In one embodiment, sensor 32 may measure or otherwiseobtain status information associated with an object to be monitored. Inanother embodiment, sensor 32 may measure or otherwise obtain statusinformation associated with the environment in which the object islocated. In another embodiment, sensor 32 may measure or otherwiseobtain status information concerning an environment to be monitored, inwhich case the transponder 12 and the sensor 32 are not necessarilyattached to or associated with any particular object. In any case,transponder 12 receives the status information from the sensor 32,generates a spreading code based on the sensor data as described belowwith respect to FIGS. 10-12, and transmits it via a coded transpondersignal for receipt and analysis by detector 18.

The type of sensor 32 may depend upon the particular application forwhich the system is used, and it shall be understood that the inventionis not limited with respect to the type of sensor. Sensor 32 may be, forexample, a medical device or other device for obtaining biomedicalinformation such as body temperature, heart rate, blood pressure, EKG,or other biological parameter. Alternatively, sensor 32 may be, forexample, a motion or tamper-proof sensor attached to the object. Sensor32 may also be, for example, a temperature sensing device, sensor fordetecting heat, light, motion, sound, velocity, acceleration, pressureor force, sensor for detecting airborne particles, chemicals orbiological agents, sensor for detecting underwater activity, sensor formeasuring any of a variety of weather related parameters, or virtuallyany other sensor type. Furthermore, sensor 32 may include more than onetype of sensor for obtaining the status information for multipleparameters.

Status information received from sensor 32 or by other means mayinclude, but is not limited to, biomedical information such astemperature, heart rate, blood pressure, EKG, or other biomedicalparameters associated with a human being to be monitored. The statusinformation may also include virtually any parameter associated with anobject to be monitored, including, but not limited to temperature,speed, acceleration, force, or information regarding whether the objecthas been moved or otherwise tampered with. Environmental statusinformation may include, but is not limited to the detection ofmovement, heat, light, sound, presence of airborne particles, chemicalsor biological agents, weather related parameters, and the like.

FIG. 3 shows a block diagram illustrating another embodiment of atransponder 12 that includes multiple code generators 28A-28N (“28”). Inthis embodiment, code generators 28 may each have different “chiprates.” As referred to herein, the term “chip” refers to one bit of thespreading code and the term “chip rate” refers to the rate at which aspreading code is applied. The sufficiently different chip rates of codegenerators 28 cause the spreading codes generated by code generators 28to be orthogonal and significantly reduces the multiple accessinterference between transmissions. In some embodiments the multiplecode generators 28 with different chip rates may be used alone togenerate spreading codes for transponder 12. In other embodiments,however, the multiple code generators may use different chip rates alongwith the spreading techniques described with respect to FIG. 2 toincrease the number of possible transponders 12 able to communicate withbase station 14 or increase the amount of data bits conveyed by eachcoded signal.

FIG. 4 is a block diagram illustrating another embodiment of atransponder 12. The interrogation signal is received at transponderreceive antenna 20. A filter 62 filters out frequencies around theinterrogation signal. In one embodiment, filter 62 is a bandpass filtercentered around the frequency of the interrogation signal, such as 10GHz±100 MHz, 450 MHz±10 MHz, or other suitable band pass filterdepending upon the operating frequency of system 10. In anotherembodiment, transponder receive antenna 20 may be implemented using amonopole, onmi-directional antenna having a very narrow bandwidth. Inthat case, filter 62 may not be required, resulting in a reducedtransponder size.

In the embodiment of FIG. 4, transponder 12 may include threeoperational modes—a zero power, or “coma” mode, a low power, or “sleep”mode, and an active mode. Threshold/trigger circuit indicated generallyin FIG. 4 by dashed line 22 controls these modes. At the time ofmanufacture, the transponder 12 is placed in coma mode and remains incoma mode, drawing limited power, until it is initially activated by aninterrogation signal. In this way, the before use shelf life oftransponder 12 can be maintained indefinitely. A coma circuit 64 detectsreceipt of an initial interrogation signal and sends a correspondingactivation signal to interrogation detector 66. Coma circuit 64 can beimplemented using any number of circuit elements, such as a single-shotswitch, diode, transistor or the like. The generalized function of comacircuit 64 is that of a comparator circuit in which two slightlymismatched transistor circuits have slightly different voltagesresulting from the mismatch. An input signal increases the bias on thelower voltage transistor circuit. Once the two transistor circuits havethe same voltage the activation signal is generated. In one embodiment,the user can reset coma circuit 64 to coma mode when the transponder 12is not in use to prolong battery life. In another embodiment, comacircuit cannot be reset, and the device remains in sleep mode afterinitial activation when not in the presence of an interrogation signal.

Once the transponder 12 is initially activated, it operates in “sleep”mode as a low power, passive device, drawing a small amount of poweruntil activated by an interrogation signal. When interrogation detector66 detects an interrogation pulse at the output of filter 62 or fromantenna 20, interrogation detector 66 generates a “wake up” signal tobias generator 68. Bias generator 68 then applies power to clockgenerator 23, code generator 28, and transponder signal generator 24.Transponder signal generator 24 amplifies conditions and amplifies thecoded signal from code generator 28, and boosts the power of the codedtransponder signal. The output of transponder signal generator 24 isconnected to transponder transmit antenna 34. In one embodiment,transponder transmit antenna 34 is a 5 GHz antenna which transmits a 5GHz coded transponder signal. In another embodiment, transpondertransmit antenna 34 is a 225 MHz antenna for transmission of a 225 MHztransponder signal. Again, it shall be understood that the invention isnot limited with respect to the particular frequencies used for theinterrogating and transponder signals, and that any appropriatefrequencies could be substituted for the specific embodiments describedwithout departing from the scope of the present invention.

FIG. 5 shows a digital logic implementation diagram of one embodiment ofcode generator 28. As discussed above, code generator 28 employs aspread spectrum technique using wideband, noise-like signals to increasebandwidth occupancy. The spread spectrum techniques allow signalstransmitted by each transponder to be uniquely identified among othertransponder signals transmitted at the same frequency. In this way,multiple transponder signals may occupy the same transmit frequencybandwidth with minimum interference.

In one embodiment, code generator 28 generates coded transponder signalsusing “Gold codes,” a spread spectrum communication technique known tothose of skill in the art. Gold codes are based on pseudorandom noise(PN) sequences having a period of 2^(n)−1, where n is the number ofstages in the linear feedback shift registers (LFSR) used to generatethe code. The number n is determined based on the application in whichthe system is to be implemented and the associated data or messagetransmission requirements.

In one embodiment, for example, the LFSR length may be 9-bits that maybe seeded with a data packet including four bits of address and fivebits of data, such as status or other information. FIG. 5 shows anexample implementation of a code generator 28 that generates a spreadingcode based on a 9-bit seed having a 4-bit address. Code generator 28includes upper 82 and lower 84 9-bit Galois form linear feedback shiftregisters (LFSR), each of which generates a PN sequence having a periodof 2⁹⁻¹. Upon receipt of the interrogation signal, the lower LFSR 84 isseeded with a non-zero constant and the upper LFSR 82 is seeded with the9-bit data packet (4-bit address and 5-bit data) contained in ID/datastorage 30. The last four shift registers of the upper LFSR 82 areseeded with the 4-bit address from ID/data storage 30. As describedabove, this 4-bit address is unique to each transponder so as to definea set of spreading codes uniquely associated with transponder 12. Inother words, the transponder 12 is allocated all possible spreadingcodes (2^((n-k))) that may be generated with k fixed address bits, inthis case 32 (2⁽⁹⁻⁴⁾=2⁵). If more transponders are deployed, the systemmay require a larger number of uniquely assigned address bits.Similarly, fewer transponders may result in the system assigning fewerbits of the n-bit LFSR seed as address bits.

The other five shift registers of upper LFSR 82 receive data fromID/data storage 30. The data received by these five registers variesdepending on the data to be sent. The two PN sequences generated by theupper LFSR 82 and the lower LFSR 84 are combined using an exclusive-ORbinary logic gate (i.e., modulo-2 added) to produce the resulting Goldcode sequence at output 86. It shall be understood that although a 9-bitGold code is shown in FIG. 5, the invention is not limited in thisrespect. When a smaller or larger number of bits for the LFSR seed isdesired, a code generator implementing a shorter or longer period Goldcode could be used in place of the specific embodiment shown in FIG. 5without departing from the scope of the present invention. For a 17-bitLSFR seed, for example, a Gold code having a period of 2¹⁷−1 could beused. An n-bit LFSR seed would have a corresponding Gold code of periodof 2^(n)−1. More detail on Gold codes and their implementation can befound in Gold, R. “Optimal Binary Sequences for Spread SpectrumMultiplexing” IEEE Transactions on Information Theory volume IT-13 pp619-621 October 1967, and Gold, R. “Maximal Recursive Sequences with3-Valued Recursive Cross Correlation Functions” IEEE Transactions onInformation Theory volume IT-14 pp 154-156 January 1968.

In addition, although FIG. 5 is described with respect to Gold codespread spectrum techniques, it shall be understood that the invention isnot limited to the use of Gold codes. The techniques of the inventionmay be implemented using any orthogonal signals. The orthogonal signalsmay be binary, such as Gold codes or Kasami codes, or multi-level (i.e.,non-binary) spreading codes.

FIG. 6 is a flow diagram illustrating exemplary operation of atransponder 12 generating and transmitting a coded transponder signal.Initially, the transponder 12 receives an interrogation signal frominterrogator 16 (100). Transponder 12 activates its circuitry inresponse to receiving the interrogation signal (102). For example,transponder 12 may generate a “wake up” signal and apply power to clockgenerator 23, code generator 28, and transponder circuit components.Clock generator 23 then generates a transmit clock based on theinterrogation signal (104). For example, clock generator 23 may be aclock divider, such as a divide-by-n circuit, that generates a clockhaving a frequency of 1/n the frequency of the interrogation signal.Code generator 28 is then seeded with the address and/or data bits fromID/data storage 30 (106). Code generator 28 next generates a spreadingcode (108). Transponder signal generator 24 modulates the spreading codewith the carrier frequency (110). The resulting coded transponder signalis transmitted via transponder transmit antenna 34 (112).

FIG. 7 shows a block diagram of interrogator 16. Interrogator may beco-located with a detector 18 in a base station 14 as shown in FIG. 1,or may be located remotely from detector 18. Multiple interrogators 16may also be used. Interrogator 16 includes transponder poller 40 and aninterrogator transmit antenna 16. Transponder poller 40 generates ageneric RF interrogating pulse at the base station signal frequency,such as 10 GHz, 450 MHz, or other suitable frequency depending upon theparticular application in which the system is to be used. Thisinterrogating pulse activates each transponder 12 in the system, causingthem to generate and transmit corresponding coded transponder signals.

FIG. 8 shows a block diagram of detector 18. Detector 18 may beco-located with an interrogator 16 in a base station 14 as shown in FIG.1, or may be located remotely from the base station 14 and/or theinterrogator 16. Also, multiple detectors 18 may be employed to increasethe range of the system 10. Incoming signals are received by detectorreceive antenna 44, which may be, for example, a parabolic reflector orother type of receive antenna of a type well known to those of skill atthe art. The received signals are conditioned and filtered by analogsignal conditioning and filtering module 45, converted to digital formby A/D converter 46 and stored in memory 48.

After interrogation, detector 18 examines incoming received signals forpresence of any of the possible coded transponder signals. Theinterrogator 16, detector 18 and the transponders 12 communicateasynchronously. Thus, detector 18 has no information concerning thestart and stop time points (in other words, neither the time-of-arrivalnor the phase) of any transmitted spreading code within the receivedsignal. In other words, detector 18 determines presence of a codedtransponder signal without having any information concerning thetime-of-arrival or the phase of any coded transponder signal present inthe received signal.

To detect presence of any one of the available spreading codes, detector18 includes a processor 50 that asynchronously examines incoming signalsto determine whether the received signals include any one of thepossible spreading codes. Because a coded transponder signal may occuranywhere in a given received signal, processor 50 includes a correlationmodule 62 which performs a sliding correlation of the received signalwith a series of ideal correlators placed at every possible shift alongthe time axis for each of the possible (2^(n)) spreading codes. In oneembodiment, the full correlation is calculated using Fourier transformmethods. The full correlation yields a long vector of values, onecorrelation value for each possible time shift of the correlator, whichmay be, for example, the number of sample points in the received signal.The decision of whether or not a particular code is actually presentwithin the received signal may be based on the maximum or peakcorrelation value for that code. If the maximum correlation valueexceeds a certain threshold, the code is determined to be present (seeFIGS. 11A and 11B).

Processor 50 retrieves a sample of the received signal from memory 48 aswell as an ideal correlator from ideal correlator storage 54. Idealcorrelator storage 54 stores the series of ideal correlators placed atevery possible shift along the time axis for each of the possiblespreading codes. If transponders 12 and detector 18 communicate using ann-bit data packet, for example, ideal correlator storage 54 stores 2^(n)ideal code correlators. For a 9-bit data packet, for example, idealcorrelator storage contains 512 (2⁹) ideal code correlators. Each idealcode correlator is an ideal representation of a signal that correspondsto a particular spreading code. Correlation module 64 correlates thesample of the received signal with the ideal code correlator todetermine whether the received signal includes a spreading code.

Correlation module 64 may perform the correlation using Fourieranalysis, using a match filter, or other correlation technique. In oneembodiment, the discrete circular correlation (c[k]) of the ideal codecorrelator (x[n]) and the sample received signal (y[n]) can becalculated using the discrete Fourier transforms of x[n] and y[n] viathe Fourier Correlation Theorem:c[k]=IFFT{FFT{x[n]}(FFT{y[n]})*}where FFT and IFFT refer to the discrete Fast Fourier Transform andInverse Fast Fourier Transform operations respectively and “*” denotesthe complex conjugate operator. In another embodiment the mathematicallyequivalent discrete circular correlation of the ideal code correlator(x[n]) and the sample received signal (y[n]), is calculated by:

${{Correlation}\left( {{x\lbrack n\rbrack},{y\lbrack n\rbrack}} \right)} = {{c\lbrack k\rbrack} = {\sum\limits_{n = 0}^{N - 1}{{x\left\lbrack {\left( {n + k} \right){{mod}N}} \right\rbrack}{y\lbrack n\rbrack}}}}$for  k = 0  to  N − 1where the integer k is the relative shift between the two signals, “mod”refers to the modulo operation, and N is the length of the sampledsignals. In one embodiment, each ideal code correlator may be stored asa Fourier representation of the ideal signal in order to reduce thenumber of operations performed by correlation module 64. Correlationmodule 64 may, in some instances, compute a normalized correlation. Forexample, processor 50 may normalize the correlation between the receivedsignal and the ideal code correlator to a value between zero and one.

Correlation module 64 applies detection criterion to the correlation todetermine whether the received signal is from one of the transponders 12(see FIGS. 11A and 11B, for example). The detection criterion may, forexample, be a threshold detection criterion. For example, if correlationmodule 64 computes a normalized correlation between zero and one,correlation module 64 may determine that a spreading code exists if thecorrelation is above some defined fraction of the maximum value. In oneembodiment, the detection criterion may be time sensitive. For example,correlation module 64 may lower the threshold detection criterion when acorrelation occurs later in time with respect to the interrogationsignal. The time sensitive detection criterion may increase thelikelihood of detecting coded transponder signals 12 whose signalamplitude has decreased due to greater distance from detector 18.

If correlation module 64 detects existence of a spreading code from thecorrelation, processor 50 accesses a code allocation lookup table 52(see FIG. 9) to identify the transponder 12 from which the signal wassent as well as the data conveyed by the coded transponder signal. Codeallocation lookup table 52 maps the possible spreading codes to theappropriate source transponder. In addition, code correlation lookuptable 52 indicates which of the bits in the spreading code contain dataand which are address bits. Correlation module 64 decodes the codedtransponder signals and stores the transmitted data in memory 48.Correlation module 64 may further display the received monitoringinformation on display 56.

Regardless of whether correlation module 64 finds a match via thecorrelation, correlation module 64 retrieves another ideal correlatorfor a different spreading code from ideal correlator storage 54 andperforms a correlation on the same sample of the received signal.Because transponders 12 and base station 14 communicate asynchronously,more than one spreading code may be present in each acquired sample ofthe received signal. Thus, processor 50 compares each sample of theincoming signal with all of the ideal correlators in ideal correlatorstorage 54. Correlation module 64 continues to correlate the sample ofthe received signal until all of the ideal code correlators have beencorrelated with that sample of the incoming signal. Correlation module64 acquires and correlates each sampled signal with all of the idealcode correlators, storing and/or displaying the monitored informationreceived in any coded transponder signals.

Correlation module 64 may be embodied as one or more devices thatinclude logic circuitry to carry out the functions or methods asdescribed herein. The logic circuitry may include a processor that maybe programmable for a general purpose or may be dedicated, such asmicrocontroller, a microprocessor, a Digital Signal Processor (DSP),Application Specific Integrated Circuit (ASIC), and the like. Processor50 is not necessarily associated with any particular computer or otherapparatus, but may be carried out by various general-purpose orspecialized machines. The instructions may be distributed among two ormore media and may be executed by two or more machines. The machines maybe coupled to one another directly, or may be coupled through a network,such as a local access network (LAN), or a global network such as theInternet.

Software running in processor 50 and in correlation module 64 may beused for performing the continuous signal correlation and detection oftransponder signals described above. The software may be stored in acomputer-readable medium that includes instructions for causing aprogrammable processor to carry out the methods described above. Thecomputer-readable medium may include but is not limited to random accessmemory (RAM), read-only memory (ROM), non-volatile random access memory(NVRAM), electrically erasable programmable read-only memory (EEPROM),flash memory, magnetic or optical media, or the like. The instructionsmay be implemented as one or more software modules, which may beexecuted by themselves or in combination with other software.

As illustrated in FIG. 8, detector 18 may be (but need not necessarilybe) configured for global monitoring of transponders 12. Processor 50may communicate via satellite uplink 80 with a network of orbitingsatellites designated generally by reference numeral 81. Each detector18 may uplink tracking and/or monitoring information received from localtransponders to one of more satellites via satellite uplink 80. Eachdetector 18 may also downlink tracking and/or monitoring informationfrom other base stations and their associated local transponders toenable tracking of transponders located in other regions. In this way, aplurality of base stations can be deployed at various locations on theearth and transponder activity within range of those base stations canbe monitored on a global basis.

FIG. 9 is a block diagram illustrating an exemplary code allocationlookup table 52 maintained by detector 18 to identify a sourcetransponder and data conveyed by a received coded transponder signal.Code allocation lookup table 52 is a table in which each row representsa set of spreading codes allocated to a transponder 12. As describedabove, each of the transponders is assigned a unique address and acorresponding fixed number of address bits 51. In the example shown inFIG. 9, the n-bit spreading code is a 9-bit spreading code with 3-bit(k=3) address bits. In the example illustrated, each of the transponders12 is assigned three fixed address bits 51, i.e., bits B6-B8, thusallowing for eight (2^(k)=2³) transponders to transmit coded signalsconcurrently. The remaining data bits 53, i.e., B0-B5, allow eachtransponder to covey six bits of information. Since each bit B0-B5 canbe set to either one or zero depending upon the data to be conveyed,each of transponders 12 is allocated sixty-four (2^((n-k))=2⁽⁹⁻³⁾=2⁶=64)spreading codes. Transponders 12 generate one of those sixty-fourspreading codes based on the data conveyed.

The example look-up table shown in FIG. 9 is illustrated for exemplarypurposes, and may be readily varied. For example, although FIG. 9illustrates a 9-bit data packet, a larger or smaller n-bit data packetmay be used depending upon the application. The division of theavailable n-bits into address and data bits may also vary depending onthe application. For example, the number of assigned address bits (k)may vary depending upon the number of transponders present in thesystem. The number of resulting data bits may also vary based on thetotal number of bits in the data packet and the number of assignedaddress bits. Furthermore, the assigned address bits need not be themost significant bits of the n-bit data packet. For example, theassigned address bits may be bits B0-B2. Moreover, the address bits neednot be consecutive bits in the n-bit data packet. For instance, thethree assigned address bits may be bits B7, B4 and B0. In addition, fora differently sized n-bit data packet, the size of code allocationlookup table 52 would be correspondingly larger or smaller.

FIG. 10 is a flow diagram illustrating exemplary operation of a detector18 decoding a coded transponder signal. Initially, detector 18 triggersa received signal acquisition (114). Detector 18 then loads an idealcode correlator from ideal code correlator storage 54 (116). Asdescribed above, the ideal code correlator is an ideal representation ofa received signal that corresponds to a particular spreading code.Detector 18 calculates a sliding correlation with the received signal(118). Detector 18 may calculate the correlation in the time domain orusing Fourier analysis. Detector 18 applies a detection criterion to thecorrelation to determine whether the received signal is from one of thetransponders 12 (120). The detection criterion may, for example, be athreshold detection criterion. Based on the applied detection criterion,detector 18 determines whether a spreading code is present (122).

Detector 18 may detect presence of a spreading code, for example, whenthe normalized correlation exceeds the threshold detection criterion.When the normalized correlation does not exceed the threshold detectioncriterion, detector 18 may determine that no spreading code is present.When a spreading code is detected, detector 18 accesses look-up table 52to identify the correct source transponder and to identify the dataconveyed (124). Detector 18 extracts and stores the address and the datainformation in memory 48 (125).

Whether a spreading code is detected or not, detector 18 determineswhether all of the possible spreading codes have been correlated (126).If all of the spreading codes have not been correlated with the signalacquisition, detector 18 loads the next ideal code correlator from idealcode correlator storage 54 (116) and goes through the detection stepswith the next ideal code correlator. When all codes have been correlated(126), detector 18 triggers the next received signal acquisition (114)and goes through the detection steps again. Detector 18 proceeds in thismanner, continuously searching for presence of all the possiblespreading codes. Detector 18 performs a sliding correlation of everypossible spreading code with the incoming received signal.

The present system allows for asynchronous communication between thetransponders and the base station. Assigning a unique set of spreadingcodes to each transponder allows multiple transponders to transmitasynchronously using the same transmit frequency with reducedinterference between transponder signals. Asynchronous analysis ofacquired signals allows base station 14 to receive and discriminatebetween the multiple transponder signals.

FIG. 11A illustrates a graph 120 of an example received signal sampleand FIG. 11B illustrates a graph 122 of a normalized correlation of theexample received signal shown in graph 120. The received signal sampleshown in FIG. 11A was sampled at a rate of 1 giga-sample per second(GSPS) over an approximately 16 μs time frame. This example uses a 9-bitdata packet and 3-bit address to produce a total of 512 possiblespreading codes. Graph 122 of FIG. 11B illustrates the correlationcalculated by processor 50. Processor 50 has normalized the correlationfor each of the 512 possible spreading codes in the system between anormalized range between 0 and 1.0. To determine whether any spreadingcodes are present, processor 50 may apply a threshold 126 to thenormalized signal. For example, processor 50 may apply a threshold 126of 0.3 to the normalized signal. In the example shown in FIG. 11B, four9^(th) order spreading codes are present in the signal, onecorresponding to each peak 124A, 124B, 124C and 124D that exceeds the0.3 threshold 126. Once the spreading codes are identified, processor 50may access code allocation lookup table 52 to identify the transponderand the data associated with each spreading code. As discussed above,processor 50 may apply a reduced threshold later in time to account forany reduction in signal amplitude for transponders that are farther awayfrom the base station.

In one embodiment, threshold 126 may decrease with time to take intoaccount decreases in transponder signal amplitude as the distancebetween transponder 12 and detector 18 increases. This may increase thelikelihood that all transponder signals sent in response to aninterrogation signal will be detected, even as their distance from thedetector increases.

Various embodiments of the invention have been described. Theseembodiments are illustrative of the practice of the invention. Variousmodifications may be made without departing from the scope of theclaims. For example, the techniques may be used in conjunction withother modulation techniques such as binary phase shift keying (BPSK),varying chip rates, or the like. Furthermore, the CDMA techniquesdescribed above may be combined with other spread spectrum techniquessuch as frequency division multiple access (FDMA) and time divisionmultiple access (TDMA). These and other embodiments are within the scopeof the following claims.

1. A transponder comprising: a clock generator that generates a transmitclock based on an interrogation signal; a code generator that generatesan n-bit spreading code from a unique set of at least two spreadingcodes allocated to the transponder from among a plurality of availablespreading codes using the transmit clock, wherein the code generatorinputs a set of address bits and a set of data bits from a storagedevice and generates the n-bit spreading code based on the set ofaddress bits and the set of data bits, wherein the unique set of atleast two spreading codes are allocated to the transponder based ontransponder identification information; and a signal generator thattransmits a transponder signal that includes the n-bit spreading code,the transponder signal having a transmit frequency determined by thetransmit clock.
 2. The transponder of claim 1, wherein the clockgenerator frequency modulates the interrogation signal to generate thetransmit clock.
 3. The transponder of claim 1, wherein the clockgenerator comprises a clock divider that divides an interrogation signalfrequency to determine the transmit clock frequency.
 4. The transponderof claim 3, wherein the clock divider comprises a divide-by-n circuit.5. The transponder of claim 1, wherein the clock generator comprises aclock multiplier that multiplies an interrogation signal frequency todetermine a transmit clock frequency.
 6. The transponder of claim 1,wherein the set of spreading codes comprise substantially orthogonalspreading codes.
 7. The transponder of claim 1, wherein the set ofspreading codes comprise binary spreading codes.
 8. The transponder ofclaim 7, wherein the binary spreading codes comprise one of Gold codesand Kasami codes.
 9. The transponder of claim 1, wherein the set ofspreading codes comprise multi-level spreading codes.
 10. Thetransponder of claim 1, wherein the address bits comprise thetransponder identification information.
 11. The transponder of claim 10,wherein the address bits define the unique set of spreading codesallocated to the transponder.
 12. The transponder of claim 1, whereinthe code generator includes an upper linear feedback shift register anda lower linear feedback shift register, each of which generates apseudorandom noise sequence.
 13. The transponder of claim 12, whereinthe pseudorandom noise sequences have a period of 2^(n)−1.
 14. Thetransponder of claim 12, wherein the pseudorandom noise sequencesgenerated by the upper and the lower linear feedback shift registers aremodulo-2 added together to produce the n-bit spreading code.
 15. Thetransponder of claim 1, further comprising a threshold/trigger circuitto detect the interrogation signal and apply power to the transponder inresponse to the interrogation signal.
 16. A system, comprising: a set ofone or more transponders, wherein each of the transponders comprises: aclock generator that generates a transmit clock based on aninterrogation signal; a code generator that generates an n-bit spreadingcode from a unique set of at least two spreading codes allocated to thetransponder from among a plurality of available spreading codes usingthe transmit clock, wherein the code generator inputs a set of addressbits and a set of data bits from a storage device and generates then-bit spreading code based on the set of address bits and the set ofdata bits, wherein the set of at least two spreading codes are allocatedto the transponder based on transponder identification information; asignal generator that transmits a transponder signal that includes then-bit spreading code, the transponder signal having a transmit frequencydetermined by the transmit clock; and a detector that receivestransponder signals transmitted by the one or more transponders andexamines the received transponder signals to detect presence of one ormore spreading codes.
 17. The transponder of claim 1, wherein the codegenerator generates the n-bit spreading code based on transponderidentification information and monitored data information.
 18. Thetransponder of claim 1, wherein the spreading code comprises an n-bitspreading code including k assigned bits that comprise the transponderidentification information, where k is greater than or equal to one andless than n.
 19. The transponder of claim 18, wherein the k assignedbits comprise a unique transponder address.
 20. The transponder of claim18, wherein the k assigned bits comprise the k most significant bits ofthe n-bit spreading code.
 21. The transponder of claim 18, wherein the kassigned bits comprise the k least significant bits of the n-bitspreading code.
 22. The transponder of claim 18, wherein at least two ofthe k assigned bits are non-consecutive bits of the n-bit spreadingcode.
 23. The transponder of claim 1, wherein the code generatorgenerates the n-bit spreading code from transponder identificationinformation and monitored data information.
 24. The transponder of claim23, wherein the monitored data information comprises one of statusinformation concerning the transponder, status information concerning anobject with which the transponder is associated, or status informationconcerning an environment in which the transponder is located.
 25. Thesystem of claim 16, wherein the detector identifies which of the one ormore transponders transmitted the received transponder signals based onpresence of the one or more spreading codes.
 26. The system of claim 16,wherein the detector identifies data conveyed by the receivedtransponder signal based on the n-bit spreading code.
 27. The system ofclaim 26, wherein each of the transponders is associated with one of anobject or an environment to be monitored, and wherein the data conveyedby the n-bit spreading code is status information concerning the objector the environment to be monitored.
 28. The system of claim 16, whereinthe detector includes a processor that performs a correlation of eachreceived signal with an ideal code correlator for each spreading code inthe system.
 29. The system of claim 28, wherein the processor comparesthe correlation to detection criterion to detect presence of any one ofthe spreading codes in the system.
 30. The system of claim 29, whereinthe detection criterion comprises a threshold detection criterion. 31.The system of claim 30, wherein the threshold detection criterionchanges as a function of time.
 32. The system of claim 28, wherein thedetector includes a code allocation lookup table that maps transpondersto their allocated spreading codes, and wherein the processor accessesthe code allocation lookup table to identify which transpondertransmitted the n-bit spreading code and to identify data conveyed bythe spreading code.
 33. The system of claim 16, wherein the transpondersand the detector communicate asynchronously.
 34. The system of claim 16,wherein the clock generator frequency modulates the interrogation signalto generate the transmit clock.
 35. The system of claim 16, wherein theclock generator comprises one of a clock divider or a multiplier. 36.The system of claim 16, wherein the clock generator comprises a clockdivider that generates a transmit clock that is approximately one-halfthe frequency of the interrogation signal.
 37. The system of claim 16,wherein the clock generator comprises a clock divider comprising adivide-by-n circuit.
 38. A system, comprising: an interrogator thatsends an interrogation signal; a set of one or more transponders, eachof which is allocated a unique set of at least two spreading codes froma plurality of spreading codes, and each of which transmits atransponder signal containing an n-bit spreading code from among theallocated spreading codes in response to the interrogation signal, eachtransponder including a code generator that inputs a set of address bitsand a set of data bits from a storage device and generates the n-bitspreading code based on the set of address bits and the set of databits, wherein the set of at least two spreading codes are allocated toeach of the one or more transponders based on transponder identificationinformation; and a detector that asynchronously examines receivedsignals to detect presence of one or more of the plurality of spreadingcodes.
 39. The system of claim 38, wherein the detector includes aprocessor that performs a correlation between a sample of the receivedsignals and an ideal code correlator for each of the plurality ofspreading codes.
 40. The system of claim 39, wherein the processorperforms the correlation using discrete Fourier transforms.
 41. Thesystem of claim 40, wherein the ideal code correlators are stored as aFourier representation of an ideal transponder signal.
 42. The system ofclaim 39, wherein the processor performs the correlation using adiscrete circular correlation.
 43. The system of claim 39, wherein theprocessor compares the correlation to detection criterion to determinewhether the sample of the received signals includes any of the spreadingcodes.
 44. The system of claim 39, wherein the detection criterioncomprises a threshold detection criterion.
 45. The system of claim 44,wherein a threshold value of the threshold detection criterion adjustsas a function of time.
 46. The system of claim 38, further comprising acode allocation lookup table that maps each of the one or moretransponders to their allocated unique set of spreading codes, andwherein the detector accesses the code allocation lookup table toidentify which of the one or more transponders transmitted the detectedn-bit spreading code and to identify data conveyed by the detectedspreading code.
 47. The system of claim 38, wherein each of the one ormore transponders includes: a clock generator that generates a transmitclock based on the interrogation signal; the code generator thatgenerates the n-bit spreading code from the unique set of at least twospreading codes allocated to the transponder using the transmit clock;and a signal generator that transmits a coded transponder signal thatincludes the n-bit spreading code generated by the code generator, thetransponder signal having a transmit frequency determined by thetransmit clock.
 48. The system of claim 47, wherein the clock generatorgenerates a frequency modulated clock.
 49. The system of claim 47,wherein the clock generator comprises a clock divider.
 50. A transpondercomprising: a clock generator that receives an interrogation signalhaving an interrogation frequency and generates therefrom a transmitclock having a transmit frequency that is a defined fraction of theinterrogation frequency; a code generator that generates an n-bitspreading code from transponder identification information and monitoreddata information, wherein the code generator inputs a set of addressbits and a set of data bits from a storage device and generates then-bit spreading code based on the set of address bits and the set ofdata bits, wherein the n-bit spreading code is generated from a uniqueset of at least two spreading codes allocated to the transponder basedon the transponder identification information; and a signal generatorthat transmits, at the transmit frequency, a transponder signal thatincludes the n-bit spreading code.
 51. The transponder of claim 50,wherein the code generator generates an n-bit spreading code, andwherein the transponder identification information comprises k assignedaddress bits within the n-bit spreading code, where k is greater than orequal to one and less than n.
 52. The transponder of claim 51, whereinthe monitored data information comprises (n-k) bits within the n-bitspreading code.
 53. The transponder of claim 50, wherein the monitoreddata information comprises one of status information concerning thetransponder, status information concerning an object with which thetransponder is associated, or status information concerning anenvironment in which the transponder is located.